There are two accepted methods of fixating long bone prosthesis during implantation. For example, with particular regard to hip stem implantation, such implants are either cemented or press-fit in place. Cementation of a hip stem has a distinct advantage of initial fixation. Press-fit stems, however, often provide for better stress distribution as well as easier revision.
Implanting a cemented hip prosthesis involves three general steps. First, the femoral canal is prepared by methods well known in the art. Second, a bone plug is introduced and cemented into the femoral canal. And last, the prosthesis is then inserted and held in place until the cement sets.
Although pressurization of the cement in the canal is achieved by forcing the cement into the canal and then inserting the prosthesis, the pressure is low. Since presently used bone cement does not adhere well to a metallic surface, state of the art techniques achieve attachment by interdigitating the cement with a roughed or machined stem surface and the rough internal bone and/or bone pores. Therefore the cement adheres mainly by mechanical means.
Some authors have stated that a cement precoat of the hip stem enhances bone strength when inserted into the cement introduced into the femur. Davies, Singer and Harris, The Effect of a Thin Coating of Polymethylmethacrylate on the Torsional Fatigue Strength of the Cement-Metal Interface, Journal of Applied Biomateria, Vol. 3, 45-49 (1992). This is apparently correct since the cement precoat is applied under pressure and is therefore forced into the hip stem pores more completely and deeper than merely by inserting a hip stem into a cement bed in the femur. However, although this precoat technique may be better than standard cementing techniques, it can still have problems due to potential debonding or delaminating of the cement--cement interface. Therefore, it is ideal to gain the pressurization of the precoat but at the time of implantation so that the cement-prosthesis and cement-bone interfaces are achieved by one continuous cemented layer.
Although cemented hip stems are the state of the art standard by which other stems are measured in terms of implantation time, without the high risk of loosening of the stem, revisions of such stems are often quite difficult. Removing cement from the femoral canal below the lesser trochanter is often a long and involved procedure.
On the other hand, press-fit stems have advantages and disadvantages over cemented hip stems. Cement is relatively weak in tension and upon failure, debris from the broken cement mantel can cause a histiocytic response. The press-fit stem eliminates the cement mantel entirely. Secondly, strain distributions in the cement mantel at the distal tip may lead to premature distal cement failure and increased wear debris leading to osteolysis, as reported in "Peak Cement Strains in Cemented Femoral Total Hips". D. O'Conner, W. H. Harris et al. Orthopaedic Research Society, Vol. 16, Sec. 1, pg. 220. There is also often a lack of initial fixation in the press-fit hip implant.